1. Field of the Invention
The present invention relates to multiphase protection circuitry for electrical power supply networks, and more particularly to such protection circuitry provided with means for dividing blocking signals in each phase, and for blocking tripping signals in each phase in the event of a fault lying behind the network component to be protected.
2. Prior Art
When a fault occurs in an electrical power supply network, an electrical travelling wave propagates therein and, because of repeated reflections in various discontinuities in the network, such as transformers, stations and the sources of the fault, the currents and voltages contain transients which are disadvantageous for the measuring relays in the relay protection devices. The dominant frequencies in such transients depend on the distance to the fault and the network configuration. Investigations have shown that, even with the introduction of transient suppression filters, it is difficult to use, for example, conventional impedance relays for relay protection devices, the operating time of which is to be less than the time of a cycle. In case of higher system voltages, greater demands are placed on the rapidity of the relay protection devices, while at the same time the damping of the travelling waves decreases. It is therefore more difficult to apply traditional measuring principles to the relay protection devices.
From, for example U.S. Pat. No. 3,956,671, it is known to utilize the direction of movement of the travelling waves at a measuring point for determining the direction to the source of disturbance. By comparing the polarities of the current and voltage waves, for example, it is possible to determine whether the fault lies ahead or behind the measuring point.
In the case of single-phase faults in networks with high system voltages, particularly in radial or thinly meshed networks, it is sometimes desirable for the relay protection devices to release only the faulty phase (so-called single-pole tripping). Since the other two phases are intact, it is easier to maintain stability in the network, at least for some time until high speed reclosing of the originally faulty phase.
In the case of a subsequent fault, that is, for example, a fault that spreads from a fault located behind the protected component and causes a subsequent fault farther on the line, it is desirable to achieve single pole tripping of the protected line or other component. Usually, the subsequent fault occurs in a phase which is different from that in which the original fault occurred, which is due to the increase in voltage in the intact phases with a resultant risk of insulation breakdown and arc flash-over. In conventional distance protection devices a large number of measuring relays are required such that subsequent faults are rapidly detected. However, it is difficult to obtain operating times less than 0.5 to 1 cycle. In protection devices measuring the direction of the travelling waves, the protection device is normally completely blocked in the event of an external fault, and thus it becomes insensitive to a real internal fault during the time the protection device is blocked.